**Core Concept**
The dairy farmer's presentation with an ulcer and central black eschar suggests a cutaneous anthrax infection, caused by *Bacillus anthracis*. This bacterium produces spores that can infect humans through direct contact with infected animals or contaminated products. Selective media are used to isolate and grow the responsible organism in the laboratory.

**Why the Correct Answer is Right**
The correct answer is based on the ability of the selective medium to inhibit the growth of other bacteria while allowing *Bacillus anthracis* to grow. The selective medium used for *Bacillus anthracis* is Blood Agar with 10% sheep's blood, which is enriched with V8 juice. V8 juice is a key component as it provides the necessary nutrients for the growth of *Bacillus anthracis*. The organism forms characteristic black colonies with a rough surface due to the production of anthrax toxin.

**Why Each Wrong Option is Incorrect**
* **Option A:** MacConkey agar is a selective medium used for the isolation of Gram-negative bacteria, particularly Enterobacteriaceae. It is not suitable for the growth of *Bacillus anthracis*.
* **Option B:** Chocolate agar is a type of blood agar that is enriched with heat-treated blood, which makes it more suitable for the growth of fastidious organisms such as Haemophilus and Neisseria species. It is not the preferred medium for *Bacillus anthracis*.
* **Option D:** Sabouraud dextrose agar is a selective medium used for the isolation of fungi. It is not suitable for the growth of *Bacillus anthracis*.

**Clinical Pearl / High-Yield Fact**
* *Bacillus anthracis* produces three major toxins: edema factor, lethal factor, and protective antigen. These toxins contribute to the pathogenesis of anthrax infection and can cause significant morbidity and mortality if not treated promptly.

**Correct Answer:** C. Blood Agar with 10% sheep's blood, enriched with V8 juice.